1. Field of the Invention
The present invention generally relates to curable compositions free of added metal catalysts. The inventive compositions are capable of curing when applied to a metal substrate. The inventive compositions provide improved thermal performance and enhanced cure strength, particularly on oily metal surfaces.
2. Brief Description of Related Technology
Curable compositions are used in a broad range of industries, including the construction, appliance and automobile industries. Typically, curable compositions used in these industries have been tailored to provide properties required for the application at hand. Such properties include, in the cured state, strength, toughness and thermal stability. In addition, adhesion to substrates constructed from a variety of different materials and oil resistance are also often desired. Adhesives may be used as high strength bonding materials including rapid-fixturing structural adhesives, and also as materials which serve primarily to seal or lock the joint between adjacent surfaces (such as threadlocking compounds for nuts and bolts) where comparatively low adhesion is adequate.
One type of curable composition, which has found wide utility in a number of industrial applications, and has been particularly important to the metal-working industry, are anaerobic adhesives. These compositions remain liquid in the presence of air, but polymerize upon the exclusion of air to form hard, durable resins with excellent adhesive properties. Since air is excluded between mating surfaces of metal articles, these adhesives are particularly useful in threadlocking applications.
In those applications where at least one substrate to be bonded contains a metal surface such as iron, copper or alloys of such metals, it has long been recognized that the metal may act to catalyze the cure of acrylic anaerobic compositions. Because the catalytic effect is not always positive (in some instances the metal substrate tends to poison the desired reaction) there have been attempts to develop compositions to beneficially exploit the catalytic action of metallic active surfaces since such compositions would be convenient and allow for a one-component adhesive. For example, Skoultchi, in U.S. Pat. No. 3,880,956 discloses anaerobic acrylic adhesive compositions which are activated by contact with metal surfaces. These compositions are single package anaerobic compositions containing diazonium salt catalysts which cure through a free radical polymerization mechanism when excluded from air or oxygen and are in contact with certain metal surfaces such as iron, copper, alloys of these metals, cadmium, chromium, nickel and zinc chromate platings. Additionally, Skoultchi, in U.S. Pat. No. 3,957,561 discloses one-package acrylic anaerobic compositions comprising at least one diazosulfone compound and o-sulfobenzimide which cure through a free radical polymerization mechanism when the adhesive is excluded from air or oxygen and is in contact with active metal surfaces.
Although these compositions are convenient to use, none of them specifically disclose the use of oily metal surfaces to effectuate cure. Oftentimes, substrates are contaminated with oil and must be degreased to facilitate bonding. Such degreasing may require suspension in solvents which may require compliance with governmental regulations or time consuming cleaning steps. Hence, there is a need for a curable composition that is oil resistant and does not require the use of added metal catalyst when used to bond metal substrates.
Further, the bonding of metal substrates used in numerous applications in the automobile and other industries often may require that the adhesive composition have excellent thermal stability. Cyanate ester resin-based compositions are known to have desirable thermal properties and show excellent adhesion to metal, especially to steel (see, e.g., U.S. Pat. No. 6,069,219 and “The Chemistry and Technology of Cyanate Esters” by I. A. Hammerton 1994 Blackie Academic & Professional for other uses and properties of cyanate esters). These resins may be produced by reacting a phenolic compound with cyanogen halide and upon curing, are known to form hard matrices through cyclotrimerization of the cyanate ester groups (see, e.g., U.S. Pat. No. 3,553,244). The cyclotrimerization produces aryoxytriazine rings that serve as crosslink sites and the cure is effected in the presence of heat by catalysts such as those described in U.S. Pat. Nos. 4,330,658, 4,330,669, 4,785,075 and 4,528, 366 which include transition metal catalysts such as Zn, Cd, Cu, and Ni salts. Organometallic catalysts have been also reported in U.S. Pat. No. 6,069,219 to effect cure of acrylic anaerobic/cyanate esters compositions. The disclosed compositions call for the addition of transition metal or other catalyst.
Maleimides have also been used in anaerobically curable compositions to impart improved resistance to thermal degradation and/or to improve strength properties at elevated temperatures (see, e.g. U.S. Pat. Nos. 3,988,299, 6,034,194, 6,034,195, and 6,391,993). Additionally, maleimides have also been used with cyanate esters resulting in compositions having good dielectric properties useful for laminate preparation (see e.g. U.S. Pat. Nos. 5,789,757 and 6,187,886).
There still, however, remains a need in the art for an oil resistant, thermally stable curable composition exhibiting high strength at elevated temperatures for use as a threadlocking adhesive that does not require the composition to include an added metal catalyst.